In a large saucepan, heat oil over medium heat. Add onion, cover, and cook until soft, about 6 minutes. Add garlic, and cook 1 minute more.
Pour 1 cup broth and the flour into a jar with a tight-fitting lid and shake to blend. Pour remaining broth and broth-flour mixture into saucepan; bring to a boil, whisking constantly. Reduce heat, and simmer 5 minutes. Stir in shrimp, and cook 3 minutes more. Stir in lemon juice, butter, and parsley. Pour over cooked pasta.